For the purpose of economising raw materials and energy, for many years attempts have been made to produce constructional components optimised for loading. More particularly in the case of motor vehicles, in which weight plays an outstanding role in fuel consumption, these efforts have led to the use of components produced from tailored blanks at different places on the motor vehicle. Such components consist of sheet metal parts of different thicknesses welded together. Of course, due to the sudden change in thickness are not at every place optimum from the aspect of subsequent loading.
To obtain components optimised for loading and free from such a sudden change in thickness, very recently tests have been carried out on so-called flexible cold rollingxe2x80x94i.e., a cold rolling of strips, wherein the nip between the rolls is changed in dependence on the travel of the strip passing through. (B. Hachmann, R. Kopp, Aachen, xe2x80x9cRolling of longitudinal sections optimised for loadingxe2x80x9d, pages 4.2-1 to 4.2-6, xe2x80x9cUmformtechnikxe2x80x9d, (=Shaping Technology), 7th Aachen Steel Colloquium, Mar. 26 to 27, 1992, Institut fur Bildsame Formgebung (=Institute of Ductile Shaping), Rhineland-Westphalia Technical University, Aachen). However, these tests have not yet led to results usable in practice. Apart from the difficulty of reaching the high rolling forces required, another difficulty is that during cold rolling the strips are subjected to different degrees of shaping over strip length. The different solidification associated therewith is undesirable for the majority of applications.
Patent Abstracts of Japan, vol. 011, no. 125. (M-582) also discloses the starting-up of a continuous strip casting installation, wherein during the start-up the nip formed between water-cooled rolls is gradually increased in size, the casting speed being simultaneously so increased as to achieve a constant thickness in the strand shells. In that process the object is not to produce strips having a thickness constantly changing in the longitudinal direction of the strip.
It is an object of the invention to provide for the production of steel strips optimised for loading a process which both allows the steel strips to be used directly and also to be further shaped without the aforedescribed disadvantages during cold rolling.
The invention therefore relates to a process for the production of steel strips optimised for loading and having a thickness continuously changing in the longitudinal direction of the strip. According to the invention such a process is characterised in that the steel strip is produced by casting between two cooled rolls with a variable nip and, in dependence on the travel of the cast strip passing through, the cooling of the rollers acting on the molten steel cast into the gusset formed by the rollers and therefore the thickness of the steel strip are so changed as to give the steel strip a length-related change in thickness of 10 to 40%.
A steel strip of homogeneous structure but having a thickness differing over its length can be produced by the process according to the invention. Such a strip provides the best preconditions for subsequent further shaping treatment either by hot rolling, or more particularly by cold rolling. Constant degrees of shaping can be achieved without problems if the strip is cold rolled.
In embodiments of the invention there are different possible ways of adjusting the thickness of the strip via the cooling of the rolls. In one possible embodiment of the invention the cooling performed by the rollers is influenced via their time of contact with the molten steel. In actual fact this can be done by adjusting the peripheral speed of the rolls and/or via the level of the molten metal. However, an alternative possibility is that the cooling is influenced via the heat flow between the rolls and the molten steel. In actual fact this can be done via the surface structure of the rolls and/or an inert gas atmosphere and/or casting oil.
Since the thickness of the strip is influenced by the cooling of the rolls and therefore via the strand shells of the strip in course of formation, a simple adjustment of the supporting force is advisablexe2x80x94i.e., the rolls can be acted upon with a constant to slightly increasing supporting force. If the strip is to become thicker, cooling is increased, with the consequence of thicker strand shells. This means that the rolls yield backwards and form a large nip. Conversely, the supporting rolls adjust if cooling is diminished, so that the strand shells become thinner. In any case this procedure ensures that the strand shells contact one another and become welded to one another at the so-called kissing pointxe2x80x94i.e., the opposite apices of the rolls.
If the cast strip is to be further reduced in thickness, it can be hot rolled. Moreover, the structure of the strip can be influenced more particularly via cold rolling and also via possible recrystallisation annealing.